The present invention relates in general to the field of surface micromachining, and in particular to a surface-micromachined rotatable member (e.g. a gear or rotary stage) having a low-contact-area hub (i.e. an axle assembly), and to a method for fabrication thereof.
Surface micromachining can be used to build up the structure of a micromechanical or microelectromechanical device layer by layer by alternately depositing and patterning a plurality of layers of polycrystalline silicon (hereinafter polysilicon) and a sacrificial material (typically a silicate glass or silicon dioxide). After the surface-micromachined structure has been built up, it can be released for operation by removing the sacrificial material using a selective etchant that dissolves the sacrificial material and leaves the various polysilicon layers intact.
A particular problem of interest for surface micromachining is that of improving the performance and reliability of rotating members. Rotating members are subject to wear and surface adhesion (also termed stiction). Lubricants are generally not available for use on a microscopic scale so that overcoming wear and surface adhesion is a major concern in the development of improved surface-micromachined devices. Surface adhesion effects become dominant at very small dimensions and must be overcome before rubbing bodies can move with respect to each other. This can affect the amount of actuation force that must be provided by a motive source (e.g. an electrostatic actuator or microengine) to drive a particular surface-micromachined device.
The present invention provides a rotatable member having a low-contact-area hub to reduce the effects of surface adhesion and friction to provide more efficient operation and improved wear resistance and reliability.
An advantage of the present invention is that a spacing between a rotatable axle and a surrounding stationary axle support of the hub can be made to be less than or equal to 0.3 microns (xcexcm) to reduce wobble or play in the rotatable member
Another advantage of the present invention is that stiction is reduced by providing a small contact area between the rotatable axle and the stationary axle support thereby reducing the actuation force for movement of the rotatable member.
Yet another advantage of the present invention is that fabrication of the rotatable member is relatively insensitive to mask misalignment for patterning the various layers.
These and other advantages of the present invention will become evident to those skilled in the art.
The present invention relates to a surface-micromachined rotatable member (e.g. a gear or rotary stage) formed on a substrate (e.g. silicon), and comprising a hub formed from at least one semiconductor layer, with the hub further comprising a stationary axle support attached to the substrate and surrounding a rotatable axle; an annulus centered about the hub and formed from the semiconductor layer; and a bridge connecting the annulus to the hub, with the bridge being formed from another semiconductor layer. Each semiconductor layer can comprise, for example, polysilicon. The axle can include a notch thereabout for supporting the axle above the substrate by engagement with a circular flange on the axle support. An air-gap spacing separating the notch from the circular flange can be, for example, 0.3 microns or less. The axle generally has a diameter in the range of 2-20 xcexcm; and the annulus, which in the case of a gear can include a plurality of gear teeth spaced about an outer circumference thereof, can have an outer diameter in the range of 20 to 1000 xcexcm. A plurality of dimples can be fabricated in the annulus to protrude below a lower surface thereof.
The present invention further relates to a method for forming a surface-micromachined rotatable member that comprises steps for alternately depositing and patterning a plurality of layers of a semiconductor (e.g. polysilicon) and a sacrificial material to build up a structure for the rotatable member which further comprises a hub having a stationary axle support surrounding a rotatable axle, and an annulus surrounding the hub and connected to the axle by an overarching bridge; and removing the sacrificial material, at least in part, by etching and thereby releasing the rotatable member for movement. The process of building up the structure for the rotatable member can include one or more chemical-mechanical polishing steps (e.g. for planarizing at least one of the sacrificial layers prior to patterning thereof).
The annulus can be formed from a pair of the semiconductor layers laminated together; and the bridge can be formed from yet another semiconductor layer of the plurality of semiconductor layers. The stationary axle support is also generally formed from the same laminated pair of semiconductor layers used to form the annulus.
To precisely locate the axle in the stationary axle support, a cavity can be etched into a first sacrificial layer, with the axle then being built up by the deposition of semiconductor material (e.g. polysilicon) into the cavity after first depositing a second sacrificial layer (generally xe2x89xa60.3 xcexcm thick) within the cavity to separate the axle from the stationary axle support. Etching of the cavity can be performed using an isotropic etching step, or using a combination of an anisotropic etching step followed by an isotropic etching step. The anisotropic etching step can comprise, for example, reactive ion etching; and the isotropic etching step comprises etching with an isotropic etchant including hydrofluoric acid (HF). The isotropic etching step undercuts the first sacrificial layer below a flanged portion of the stationary axle support so that the axle will be retained within the axle support after the etch-release step whereby the sacrificial material is removed, at least in part, by etching.
The present invention also relates to a method for forming a surface-micromachined rotatable member, comprising steps for depositing at least four layers of polysilicon on a substrate (e.g. a silicon substrate), and depositing a plurality of sacrificial layers, with one sacrificial layer being located between each adjacent pair of the layers of polysilicon; patterning each layer of polysilicon after deposition thereof and forming therefrom a hub having a stationary axle support surrounding a rotatable axle, and an annulus surrounding the hub and connected to the axle by an overarching bridge; and removing each sacrificial layer, at least in part, by etching and thereby releasing the rotatable member for movement. A second polysilicon layer and a third polysilicon layer of the at least four layers of polysilicon can be laminated together to form the stationary axle support and the annulus. The bridge connecting the annulus to the axle can be formed from a fourth polysilicon layer.
The patterning steps can include a step for forming a cavity within the stationary axle support. This can be done using one or more etching steps to etch a first sacrificial layer of the plurality of sacrificial layers. A second sacrificial layer can be then deposited within the cavity to determine a precise separation between the stationary axle support and the axle which will be formed in the cavity. The second sacrificial layer generally has a layer thickness of 0.3 microns or less to provide a low play between the axle and axle support. One or more of the sacrificial layers can also be planarized, as needed, using a chemical-mechanical polishing step.
Additional advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following detailed description thereof when considered in conjunction with the accompanying drawings. The advantages of the invention can be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.